Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate

Carosi, Rodolfo; Montomoli, Chiara; Iaccarino, Salvatore; Massonne, Hans-Joachim; Rubatto, Daniela; Langone, Antonio; Germignani, Lorenzo; Visoná, Dario (2016). Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate. Geological Society of America bulletin, 128(11/12), pp. 1571-1592. Geological Society of America 10.1130/B31471.1

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ABSTRACT
We investigated a contractional shear zone
located in central Nepal, known as Kalopani
shear zone. This high-temperature shear
zone triggered the early exhumation of the
metamorphic core in the Himalayan belt and
deeply affected the tectono-metamorphic history
of the crystalline rocks soon after the
collisional stage. Pseudosection modeling
and inverse geothermobarometry reveal that
rocks involved in the Kalopani shear zone
experienced pressure-temperature conditions
between 0.60 and 0.85 GPa and 600 and
660 °C. U-Th-Pb in situ laser ablation–
inductively
coupled plasma–mass spectrometry
and sensitive high-resolution ion microprobe
dating on monazite points to retrograde
metamorphism related to the Kalopani shear
zone starting from ca. 41 to 30 Ma. The
kinematics
of the Kalopani shear zone and
associated erosion and/or tectonics caused
the middle-late Eocene exhumation of the
Greater Himalayan Sequence in the hanging
wall of the Kalopani shear zone at least 9 m.y.
before the activities of the middle tectonicmetamorphic
discontinuity in the Greater
Himalayan Sequence (High Himalayan
discontinuity),
the Main Central thrust, and the
South Tibetan
detachment.
Structural data, metamorphic conditions,
and geochronology from the Kalopani shear
zone, compared to those of other major tectonic
discontinuities active within the Greater
Himalayan Sequence in the Kali Gandaki
valley, indicate that shear deformation and
exhumation were not synchronous all over
the Greater Himalayan Sequence but migrated
downward and southward at different
lower levels. These processes caused the
exhumation of the hanging wall rocks of the
activated shear zones. The main consequence
is that exhumation has been driven since the
middle-late Eocene by an in-sequence shearing
mechanism progressively involving new
slices of the Indian crust, starting from the
metamorphic core of the orogen and later
involving the outer portions of the belt. This
challenges the common view of exhumation
of the Greater Himalayan Sequence mainly
driven by the coupled activity of Main Central
thrust and South Tibetan detachment
between ca. 23 and 17 Ma.

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